Fuel cells and metal-air batteries have been known for many years. However, commercial exploitation has been slower than expected due to their generally bulky structures and the difficulties encountered in attaining adequate power densities and reasonably sustained performance. Accordingly, much effort has been expended in developing more compact cell designs and more efficient electrodes for service in the harsh chemical environments represented by the acid or alkaline electrolytes used therein.
Porous composite electrodes containing various electroconductive and catalytic particles have often received consideration for service as oxygen cathodes in such batteries and fuel cells. Representative cathodes of this character are described for example, in U.S. Pat. Nos. 3,385,780; 3,462,307; 3,553,022; and 3,668,014.
Although considerable progress has already been made in adapting such porous composite electrodes for use in electrochemical devices, the difficult problem of achieving and maintaining a controlled balance in permeability to both the liquid electrolyte and the oxygen containing gas has led to premature failures, such as blistering and delamination, under more demanding service conditions. For example, in metal-air batteries having cell potentials of about 2 volts, available porous, carbon based oxygen cathodes have not heretofore been capable of sustained performance at high current densities (i.e. substantially above about 400 milliamps per sq. cm.) for much more than a full hour at best. One of the most common causes of oxygen cathode failure is believed to be flooding of the porous cathode structure by electrolyte, but attainable current density can also be reduced by excessive gas percolation therethrough and/or depletion of catalytic activity therein.